Press rolls and calender rolls are critical components in a paper machine both from the point of view of the runnability of the machine and from the point of view of the quality of the product, because in a press and in a calender, the roll face is often in direct contact with the paper web. Direct contact with the face of a roll has a significant effect on the surface properties of paper, which imposes high requirements on the quality of the face of the roll. The surface properties of the roll are also critical from the point of view of the paper-making process. The wet or coated paper web must adhere to the roll in a suitable way, however, on the other hand, it must also be readily separable from the roll. It must be readily possible to doctor the roll, easy to keep it clean, and the roll must remain in good operating condition for a long period of time. Further, from the point of view of runnability, it is essential that the surface properties of the roll do not change during the process so that, for example, the separability of the web from the roll face cannot be suitably controlled. Owing to the direct contact of the roll face with the web, fibers and contaminations adhere to the roll face and block the surface layer and, consequently, the surface properties of the roll are changed. In order to amend the effect of contamination, the roll is doctored, which again imposes its requirements on the mechanical surface properties of the roll, i.e., the roll face must adequately withstand the effects of doctoring.
In a press, particularly demanding surface properties are required from center rolls in presses. At center rolls, the web is pressed against the roll face in two or more nips. Also, demanding surface properties are required from backup rolls of extended-nip presses in which the contact face between the web and the roll is longer and the nip load higher. In these cases, the tendency of sticking of the web and contamination of the roll in web contact are particularly intensive. In such positions, in particular in wide paper machines, variable-crown rolls are used. The roll face is further subjected to particularly demanding conditions when the pressing takes place at an elevated temperature either by heating the web before the center roll (e.g., by means of a steam box) or by means of the roll when the web is on the roll face (heatable center roll or so-called impulse drying).
The granite roll, which has been traditionally used in the press section of a paper machine, has been abandoned in recent years in spite of the excellent surface properties of granite. The reason has been mainly the requirements of strength and durability increased along with higher running speeds as well as the need to profile and to heat the roll. Synthetic rock rolls, which have been coated with a mixture of ceramic powder added to a hard rubber and polyurethane or some other polymer, have performed poorly because of their low mechanical strength and excessive adhesion of the paper web to the roll face.
By means of calender rolls, the surface of paper or board can be given the desired gloss, smoothness, and finished face. Traditionally, calendering has been used mainly for compacting the surface of paper, which has taken place by means of rolls which are as smooth as possible. For this purpose, chilled rolls and hard-chromium plated rolls have been commonly used. Electrolytic hard-chromium plating has been highly laborious as a process for rolls of large size, and with respect to the quality of the face to be chromium-plated, it has been highly demanding. From the point of view of the plating process, the high susceptibility of wear of the hard-chromium plated face has constituted a problem, which wear has been increased further by the micro-particles contained in the coating material, such as clay. Owing to the wear of the plated face, the capability of holding/adhesion of the face is deteriorated, and doctoring becomes more difficult and causes streaks in the web. The production of dull-finish grades by means of the prior art rolls has not been suitable because of the rapid smoothing of the face.
Owing to the circumstances mentioned above, alternative solutions for coatings of rolls have been developed both for press rolls and for calender rolls. Most commonly, at present, rolls with metal frames are used which have been coated with a metal, a ceramic, ceramic-metal, polymers or elastomers and various mixtures of the same.
Ceramic and ceramic-metal coatings and coating processes have been described, for example, in the following publications.
In Finnish Patent Application No. 853544, a roll coating is described which consists of a metal or a mixture of a metal and a ceramic material.
In Finnish Patent No. 70,273, mixtures of metal powder and inorganic material are suggested as a coating for a press roll.
On the other hand, in Finnish Patent Application No. 861803, a construction of a press roll is described, in which a metallic adhesion layer has been applied onto the face of a metal frame, which adhesion layer has a thermal expansion coefficient lower than that of the metal frame, and onto the adhesion layer, a ceramic surface layer has been applied.
In Finnish Patent No. 84,506, a press roll is described whose metallic frame cylinder is coated with an intermediate layer consisting of a composite compound made of a ceramic material and a metal, and then with a ceramic surface layer. The mixing ratio of the components in the composite compound is different in different parts of the intermediate layer in the direction of the radius of the roll.
In Finnish Patent No. 86,566, a center roll for a press is described, which has been manufactured so that the mantle of the roll frame has been coated with a corrosion-protection layer which consists of chromium-containing stainless steel as well as of a surface layer which has been prepared by thermal spraying of a powder in which the metal and ceramic phases are contained in the same powder particle.
In International Patent Application WO 93/01326, a composition of a coating for a press roll is described, which comprises a mixture of aluminum silicate and alkaline-earth metal oxide that has been plasma-sprayed onto the face of the roll.
In European Patent Application 0 657 237, thermal spraying of cobalt-containing tungsten carbide granules and nickel-containing chromium carbide granules onto the roll face is described.
In Finnish Patent No. 89,950, a press roll for a paper machine is described, in which the mantle of the metal core of the roll has been coated with a metal layer that contains molybdenum-based and nickel-based metal alloys. Onto the metal layer, a ceramic coating is prepared by means of thermal spraying. Suitable ceramic compounds are grey aluminum oxide (95% Al.sub.2 O.sub.3 --2.5% TiO.sub.2), white aluminum oxide (99% Al.sub.2 O.sub.3), titanium dioxide (TiO.sub.2), etc. and mixtures of same.
In European Patent Application 0 481 321, a press roll is described in which an intermediate layer made of a molybdenum-based or nickel-based alloy has been applied onto the metal core, for example, by plasma spraying, and onto this layer a ceramic layer has been applied by plasma spraying, which layer consists of metal oxides or of mixtures of same. Finally, the roll is coated with an organic polymer to fill the pores in the ceramic coating.
International Patent Application WO 96/41918 is an example of a hot press roll having a coating prepared by means of thermal spraying of a metal-ceramic and a mixture of a ceramic and a certain metal alloy.
In Finnish Patent No. 92,609, a method is described in which a metal-ceramic face is sprayed onto the face of a hot-glaze calender roll or a calender roll for a machine stack, which rolls are supposed to increase the gloss of paper. The metal-ceramic mixture consists of tungsten carbide and cobalt or a nickel-chromium alloy. After coating, the face is finished by grinding.
In Finnish Patent No. 80,097, a method is described in which the roll is coated with a mixture which consists partly of a metal and partly of a ceramic material, so that the outer surface is composed of carbide-rich areas and matrix areas placed between the carbide-rich areas. Chemically, the coating on the roll is an alloy of tungsten, chromium and carbon, or an alloy of tungsten carbide, tungsten, cobalt, chromium, and carbon.
A heatable calender roll with a ceramic coating is described in European Patent No. 0 598 737, wherein a face as smooth as possible is aimed at, and a press roll with a ceramic or cermet face is described in European Patent No. 0 597 814.
It has, however, been noticed that the prior art ceramic-coated rolls mentioned above and currently available on the market involve a number of problems, deficiencies or limitations, which have become ever more critical when the running speed of the paper machine becomes higher and when the basis weight of the paper becomes lower. The process of detaching of the web and the doctoring quality are more difficult to control. These drawbacks are particularly problematic in the case of center rolls in presses and backup rolls in extended-nip presses. The Cr-oxide and Al-oxide based coatings currently in use involve drawbacks with respect to their mechanical and chemical strength. Chromium oxides are hard, but their toughness is poor, and mechanical damage tends to arise. In mechanically highly abrading conditions, the roll face is worn and ground smooth, in particular in the case of Al.sub.2 O.sub.3 -based ceramics, and Al.sub.2 O.sub.3 -based ceramic coatings do not endure doctoring with a steel blade. The resistance of ceramic coatings to chemical strains is deficient, which results in damage to the surface layer of the roll, such as corrosion and delamination. In particular, Al.sub.2 O.sub.3 -based ceramic coatings do not endure washing with lye.
Ceramic coatings have often been thermally sprayed onto the roll face, which unavoidably results in porosity of the face. Owing to the porosity, agents that produce corrosion have access to the boundary face between the roll and the adhesion coating unless the tightness of the adhesion and corrosion-protection layer is adequate. Flaws in the adhesion layer placed under the ceramic layer may result in corrosion in the roll under the coating and thus, in destruction of the whole roll during a long period of time. Difficulties are further caused by the material gathering in the face of the roll, which tends to block the pores in the surface layer, in which connection the properties of adhesion of the roll are changed.
The face of a roll must endure high linear loads, which strain the coating. Besides hardness, toughness and resistance to wear are also required from a coating. Also, the coating must be easy to repair, it must tolerate variations of temperature very well, and it must operate in a wide range of temperatures in a range of from about 10.degree. C. to about 250.degree. C. and under a nip pressure of from about 5 MPa to about 50 MPa. The face of the roll must be sufficiently hard to endure the abrading effect of the filler agents in the paper, the abrading effect of a doctor, and the effect of a corroding environment. Moreover, a roll must have the necessary surface properties for keeping the roll clean, for adhesion and separation of the paper web, such as, among other things, suitable hydrophily. Also, the coating must retain its original roughness, i.e., the Ra value, as long as possible. The face of a roll must be capable of producing the desired properties, such as, for example, uniformity of quality, low gloss (so-called dull finish), and good smoothness, or good gloss (not dull) for the paper, in particular in calender applications.